Fiber mode calculator form
Enter waveguide geometry and refractive data to estimate normalized frequency, cutoff wavelength, acceptance angle, and guided LP mode support.
Example data table
These examples show how geometry, index contrast, and wavelength shift the normalized frequency and change the expected mode population.
| Fiber case | Core diameter (µm) | Wavelength (nm) | n₁ | n₂ | Profile | NA | V | Estimated modes |
|---|---|---|---|---|---|---|---|---|
| Telecom narrow-core | 9.0 | 1310 | 1.450 | 1.444 | Step-index | 0.132 | 2.844 | 4.04 |
| LAN multimode | 50.0 | 850 | 1.480 | 1.460 | Step-index | 0.242 | 44.812 | 1004.04 |
| Graded distribution | 62.5 | 1300 | 1.470 | 1.457 | Graded-index | 0.195 | 29.462 | 217.01 |
Formula used
These formulas are practical engineering approximations. Exact mode solving requires vector boundary conditions and numerical eigenvalue methods.
How to use this calculator
- Enter the fiber core diameter and operating wavelength in the requested units.
- Provide the core, cladding, and outside-medium refractive indices.
- Choose whether your fiber behaves more like a step-index or graded-index profile.
- Select a target LP mode to check whether it is above cutoff.
- Set a single-mode design margin for more conservative screening near V = 2.405.
- Press the calculate button to view the result summary, mode table, graph, and export options above the form.
FAQs
1. What does the V-number represent?
The V-number is the normalized frequency of the fiber. It combines core radius, wavelength, and numerical aperture to estimate whether modes can propagate and how many are likely guided.
2. When is a fiber considered single-mode?
A circular step-index fiber is commonly treated as single-mode when V stays below 2.405. Below that threshold, only the fundamental LP01-like mode remains guided.
3. Why does wavelength change the mode count?
V is inversely proportional to wavelength. Increasing wavelength lowers V, which can push higher-order modes below cutoff and reduce the total number of guided modes.
4. Why are step-index and graded-index mode estimates different?
The field distribution changes with index profile. A graded-index design redistributes rays and wave paths, so the approximate supported mode count is lower than the comparable step-index estimate.
5. Are the LP cutoff values exact for every fiber?
No. They are useful approximations for circular weakly guiding fibers. Real fibers can shift cutoff through bending, profile details, dispersion, stress, and vector effects.
6. Why must the core index exceed the cladding index?
Guiding relies on total internal reflection or an equivalent waveguide confinement condition. Without a higher core index, the fiber cannot confine light in the usual guided manner.
7. What does cutoff wavelength mean?
It is the wavelength where the V-number reaches the selected cutoff threshold. Above that wavelength, the corresponding higher-order mode stops propagating as a guided mode.
8. Can this calculator replace a full optical mode solver?
No. It is excellent for screening and early design decisions, but final verification should use a full vector solver or measured fiber data.